During the combustion of diesel fuel in vehicles, a high degree of nitric oxides is formed, which cannot, or only unsatisfactorily, be filtered by means of the conventional catalytic converter technology. Hence, SCR catalytic converters, which are based on selective catalytic reduction, have already been used for some time for the purpose of reducing the nitric oxides. For effecting said selective catalytic reduction, a urea solution is injected into the exhaust gas flow, so that the most of the nitric oxides will be converted into nitrogen and hydrogen. This technique and the urea solution are also known under the brand name AdBlue. The urea solution is stored in special containers or tanks from where it is supplied by means of metering pumps via lines and tubes to injectors for injection into the exhaust gas flow.
The material that has hitherto been used for the lines and tubes has normally been polyamide, preferably PA 12. The polyamide tubes are preformed so that, once installed, they will retain their curvatures and geometries comparatively stiffly. In addition, polyamide tubes and lines may also be coated with additional materials, which make the tubes and lines resistant to the urea-containing liquid. In the case of known polyamide tubes, a so-called hydrolysis takes place, in the course of which the material is chemically broken down in contact with water. Only one third of the urea solution consists of urea; the balance is water. Hence, polyamide has a non-proven, permanent resistance to the urea solution, so that coated polyamide is preferably used.
The components used for the purpose of selective catalytic reduction, such as the tubes and lines for filling, emptying, feedback and also for venting the tanks must, especially in the case of passenger cars, be configured such that they have many angles and take up little space. Insofar, complex geometries of the lines and tubes are obtained. As has already been mentioned hereinbefore, the polyamide tubes are preformed for this purpose, so that the tubes can easily be adapted to the respective situation. During installation, the polyamide tubes are, due to the space available, bent, stretched or compressed, so that stress whitening may occur. This has the effect that the tubes are weakened, even though they do not break.